Image sensors of the named kind are used, for example, in digital still cameras and moving image cameras (movie cameras) for taking single images or a sequence of moving images. The image signals of these images are composed of pixel signals which are generated by the pixels of the image sensor which are typically arranged in rows and columns. Due to quality fluctuations in the production of the image sensors, individual pixels can be defective. In this respect, any deviation of a pixel from a standard specification is called a defect which has the result that the pixel, and in particular pixel signals generated by the pixel, is/are unusable or can only be used with limitations. For example, with a conventional image sensor for a moving image camera typically having several millions of pixels, 20 pixels are unusable and 3000 pixels can be used with limitations, while the remaining pixels can be used without limitations.
Especially in the area of professional moving image cameras which are also used for cinema productions, the differences caused by defective pixels cannot be tolerated even with a relatively small number of defective pixels in comparison with the total number of pixels. For, on the one hand, even differences of individual pixels also have a disturbing effect in the postprocessing of the image signals formed from the pixel signals. On the other hand, in particular on a projection of the image signals onto large cinema screens, even differences of individual defective pixels can become visible. The yield is therefore disadvantageously small in the manufacture of image sensors for cameras having such a high quality requirement.
To the extent that it appears tolerable in an individual case for a relatively small number of defective pixels, the pixel signals of all pixels of an image sensors which are identified as defective are discarded and new pixel signals are estimated for the defective pixels by interpolation of pixel signals of adjacent pixels. In this respect, a respective estimated pixel signal should come as close as possible to a pixel signal which a pixel usable without limitation would have produced at this position so that where possible a disturbance can no longer be perceived in the image signal after the correction. Since in each case all the defective pixels are uniformly discarded, that is both unusable pixels and pixels which can be used with limitations, the number of pixel values which have to be calculated by interpolation is always the same, which causes a high correction effort. In addition, image information which may possibly be usable is discarded by the discarding of the pixel signals of pixels which can be used with limitations so that the image quality may be impaired to an unnecessary degree.
For a more differentiated correction, the pixels of an image sensor can be differentiated in accordance with a defect characteristic as to whether they are unusable, can be used without limitations or correspond to one or more predefined defect classes. Different degrees of limited usability of respective pixels can then be detected by these defect classes. The unusability and/or the unlimited usability of a pixel can in particular also be described as a respective such defect class. This categorization into defect classes makes it possible also to treat pixels with defects of different degrees differently. Pixels within the same defect class are in contrast still corrected uniformly.
However, in this procedure, the differences caused by defective pixels with respect to their degree and/or their perceptibility in the image signal are not necessarily constant. For example, the difference of a defective pixel can depend on the intensity of its lighting (light or dark). The amount by which a difference in the pixel signal of a defective signal can be perceived and which is thus in need of correction can in particular depend on the environment of the pixel, that is on the pixel signals of adjacent pixels. It is therefore not absolutely necessary in equal measure for all pixels of a defect class to correct them for an improvement of the image quality. Potential for a further reduction in the processing effort therefore remains unused when only one defect characteristic is considered. Furthermore, a correction itself always suffers from uncertainty. A correction which is only orientated on a defect class associated with the respective pixel can therefore at times even worsen the image quality.